The sun's gravity also contributes to the tides, but its effects are smaller and serve mostly to increase or decrease the height of the tides.

What is kinetic energy?

    Kinetic energy is the energy of a body related to its motion. The magnitude of the kinetic energy depends on both the mass and the speed of the object, and it can be expressed by the following equation:

E = 1 mv²
      2
Where E is the kinetic energy, m is the mass of the object, and v² is its speed squared (multiplied by itself).

What is torque?

    Torque is twisting or spinning force that causes an object to turn about its axis of rotation. When you twist the top off a jar you exert torque on the jar and cause it to rate of change in angular velocity. The top of the jar begins to rotate faster and faster in the direction of your torque. Similarly, when you spin a toy top, you exert torque so that the top undergoes an angular acceleration. 

What is the relationship between wavelength, frequency, and amplitude?

    Wavelength is the distance between two consecutive peaks of an emitted wave. Frequency corresponds to the number of wavelengths that pass by a certain point in space in a given amount of time. All electromagnetic waves travel at the same speed, so, for a given period of time, a greater number of short wavelength waves will pass by a point in space than will long wavelengths. This means that shorter waves have a higher frequency that longer ones. 
    The relationship between wavelength, speed, and frequency is expressed by the equation: c = If, where c is the speed of a light wave in meters per second (3x108 meter per second in a vacuum), I is the wavelength in meters, and f is the wave's frequency in Hertz (cycles per second or Hz). 

    The amplitude of an electromagnetic wave is the height of the wave, measured from a point midway between a peak and a trough to the peak of the next wave.

What is the Doppler effect?

    Named after Austrian physicist Christian Johann Doppler, who first described this principle, the Doppler effect refers to the observed frequency changes in waves due to a source that is moving toward or away from an observer, or from an observer moving toward or away from the source. It explains why, to a person standing at a train station, a train's whistle will rise in pitch as the train approaches the platform (sound waves are compressed), and fall in pitch as it moves away (sound waves are elongated). Observers on a speeding train will hear a similar change in pitch from a factory whistle as they pass it. This is also the Doppler effect at work.

What is resonance?

    In physics, resonance refers to the oscillation of a system at its natural frequency of vibration when triggered by a stimulus with an appropriate frequency. A common example of this phenomenon occurs in music. If you strike a note on a piano that is the same frequency as one of the strings of a nearby guitar, the string on the guitar will resonate (vibrate), even though it has not been plucked. Resonance can occur in acoustical systems, electrical systems, mechanical systems, and on the atomic level.

What is ultrasonic?

    Ultrasonic refers to sound vibrations with frequencies above the range audible to the human ear-above a frequency of 20,000 Hz (cycles per second). Nocturnal animals, such a bats, use ultrasonic vibrations to guide their movements in the dark. 
Ultrasonic waves are used in sonar and as a diagnostic tool to produce images of the internal body. 

What is the sonic boom?

    A plane that flies faster than the speed of sound (760 mph) travels faster than the sound produced by its jet engines. As a result, the engine's sound waves lag somewhat behind the plane, spreading out in a conical shape. Since the plane's sound is  " contained" within this trailing cone, you will not hear the plane although you may see it. When the cone does reach you, a large amount of sound hits you all as once in a thunderous clap called a sonic boom. The cone itself is a kind of shock wave, with an abrupt increase in pressure right at its edge (sound and pressure are closely related). 

    Note that the sonic boom occurs when the plane's shock wave reaches your ears, not when the plane breaks the sound barrier. 

How is light emitted?

    Light is emitted by electrons circling the nucleus of an atom. The electrons, circling the nucleus in specific patterns called orbitals, possess a quantity of energy at a given instant of time. Those circling in orbitals closest to a nucleus have less energy than those in the outer orbitals. The amount of energy associated with each orbital is called an energy level. If an electron in a lower energy level gains enough energy, it will jump to a higher level. When this happens, the electron is said to be 'excited." 

    As an excited electron moves, it eventually loses energy and falls back to a lower orbital. The energy an electron releases is equal to the difference between the energy levels of the higher or lower orbitals. It is this quantum of released energy, in the form of a photon that we see as emitted light.

Why do some things glow in the dark?

    Substances that glow in the dark store the energy they get from visible light. This stored energy is gradually released and converted back into light, which causes the substances to glow dimly.
On a molecular level, the energy from the light excites electrons, which then become trapped in a high-energy state. It takes a long time for the electrons to return to their original states, but they are able to "calm down" by emitting light. Eventually, the electrons will return to their original state and the glow will fade.

What distinguishes ultraviolet light from infrared light?

    The human eye can perceive only a tiny fraction of the electromagnetic spectrum (the entire range of electromagnetic radiation classified according to wavelength). The portion that can be seen by humans is called "visible light." Just above visible light on the spectrum is ultraviolet light, which has a shorter wavelength, and thus a higher frequency than visible light. Infrared light exists just below visible light on the spectrum, and it has a lower frequency, and thus a longer wavelength, than visible light.

    Ultraviolet rays are sometimes used in the treatment of skin conditions, and they are also used to kill bacteria and sterilize utensils. Infrared rays can be applied in the treatment of sports-related injuries due to their ability to warm muscles.

What is polarized light?

    Polarization refers to the orientation of the electric field in an electromagnetic wave. A wave whose electric field is oscillating in the vertical direction is said to be vertically polarized. Such waves interact with matter differently than waves polarized in the horizontal direction.

    The electric field of light waves from the sun vibrates in all directions, so direct sunlight is called unpolarized - though, in truth, the light e see is partially polarized by the earths atmosphere. Polaroid sunglasses are able to block light that is horizontally polarized and therefore reduce glare from polarized sunlight reflecting off surfaces.

What is a mirage?

    A mirage is an optical phenomenon. Mirages can be divided into two groups: inferior and superior. An inferior mirage occurs when a layer of air close to the ground is heated more than the air above it. Light rays reflecting off the ground must then pass through two layers of air of varying temperature and density; a hot and relatively low - density layer and a relatively cool, high - density layer. The boundary between these two layers produces a lens-like effect, causing the light from the ground to be refracted (bent). An example of an inferior mirage is what happens on a paved road in the heat of the summer sun; you see what appears to be a river of water running over the road. 
A superior mirage forms when a layer of air next to the ground is much cooler that the air above it. Light rays from an object are refracted such a way that an object appears to be suspended in air above its actual position. This phenomenon is sometimes referred to as "looming". 

Why is "c" the symbol for the speed of light?

    The symbol "c" has been used universally to represent the speed of light (in a vacuum) since the early twentieth century. Einstein used other symbols, such as "V", in his early relativity papers, but most books on relativity used C. 
    The symbol most likely comes from the first letter of the Latin word celeritas, meaning "swift". The English word celerity, a rare synonym for speed, as well as the words accelerate and decelerate are derivatives from the same Latin root. 

How can light be redirected?

    Light can be redirected in three ways: reflection, refraction, or diffraction. Reflection occurs when light bounces off a surface like a mirror. Refraction refers to the bending of light as it passes through different media. Diffraction is a change in the direction and intensity of a group of waves after passing by an obstacle or through an aperture (opening).

What is the difference between radar and sonar?

    Radar is an acronym for "radio detecting and ranging." Radar works by scanning surrounding space using high-frequency r radio waves. These radio waves are sent out from a transmitter and are bounced back toward the transmitter by the first object they encounter in their path. The reflected beam is then picked up by a receiver. The strength and direction of the beam gives information as to the size, distance, altitude, etc., of the object. 
Sonar is an acronym for "sound navigation and ranging". Sonar determines the location of objects based of the reflection of sound waves. It was originally developed to detect icebergs after the dinking of the Titanic in 1912

What is Charles' Law?

    Jacques Charles was an eighteenth century balloonist who conducted experiments on the relationship between the temperature of a gas and its corresponding volume. The physical principle bearing his name states that the volume of a gas equals a constant value, multiplied by its temperature (in degrees Kelvin). In other words, the volume of a gas is directly proportional to its temperature. This law is expressed by the equation: 
V = kT. V is the volume of the gas, k is the constant, and T is the temperature. 

What is heat?

    Heat is related to the motion or vibration of molecules. All objects contain some quantity of thermal energy, which is the sum of the kinetic energies (vibration or movement) of its atoms and molecules. The hotter the object, the more thermal energy or movement is present in its atoms.

What is the Bernoulli effect?

    Circa 1730, Daniel Bernoulli, Swiss mathematician, was first to describe the Bernoulli effect. Bernoulli discovered that the faster fluid (defined as anything that flows, such as air, water, or a gas) moves over a surface, the less pressure it exerts on that surface. If the total energy in a steadily flowing system is constant along the flow path, then an increase in the fluid's speed must be matched by a decrease in its pressure.
A common example of the Bernoulli effect in action is the flight in an airplane. An airplane wing is designed to permit air to flow more rapidly over its upper surface than over its lower surface; thus the pressure on the top of the wing is lower than on the bottom. This pressure difference provides the lift that sustains the plane in flight.

What is a British thermal unit (Btu)?

    A Btu is a unit of heat energy. It is the amount of heat required to raise the temperature of one pound of water from 60°F to 61°F at a constant pressure.

What is radioactivity?

    Radiation includes all forms of energy that travel through space as rays. Radioactivity, however, is the spontaneous breakup or decay of certain kinds of heavy atomic nuclei. This breakup is accompanied by the emission of particles and energy. The energy (or radiation) emitted by these unstable nuclei are capable of ionizing atoms or molecules, causing them to gain or lose electrons and take on positive or negative charge. Radiation's impact on molecules such as DNA makes this form of energy a biological hazard in the case of prolonged or intense exposure. 

What is the difference between nuclear fusion and nuclear fissions?

    In fusion, nuclei collide so fast that they stick together and emit a great deal of energy. This process occurs, for instance, when lightweight hydrogen nuclei combine to form the somewhat heavier nuclei of helium. Nuclear fissions occurs when the nuclei of very heavy atoms (such as uranium and plutonium) capture neutrons and become unstable. They break apart into nearly equal parts (usually barium and krypton) releasing energy as they decay.

What was the Manhattan Project?

    The Manhattan Project was the code name for the effort to build the first atomic bomb in the United States during World War II. Many of the most notable quantum physicists of the day were involved in this project, including Robert Oppenheimer, Richard Feynman, and Enrico Fermi. 

What is the Heidelberg uncertainty principle?

    A theory formulated by German physicist Werner Heisenberg in 1927, the uncertainty principal states that it is impossible to know complementary parameters simultaneously, such s the position and momentum of a particle (photon, electron, proton, positron, etc.) with perfect accuracy. Furthermore, the principal states that as one parameter is measured more accurately, the other one will become less precise. 

What is antimatter?

    Antimatter consists of elementary particles that have the mass of ordinary particles - such as electrons, protons, or neutrons-but opposite electric charges. These antiparticles are called positrons, antiprotons, and antineutrons.

What is the black hole? 

    Black holes are cosmic bodies of extremely intense gravity from which nothing, not even light, can escape. They are formed when matter undergoes extreme compression, such as during the death of a massive star. When such a star has used up its internal fuel supply, it becomes unstable and collapses inward upon itself. The crushing weight of matter falling in from all sides compresses the dying star to a point of zero volume and infinite density, which is called a singularity. 

What is a wormhole?

    A wormhole is a distortion in space-time that acts like a "tunnel" or "passageway", connecting two distant points in the universe. Suppose you wanted to travel to the nearest star. It would normally take more than four years, traveling at the speed of light, to reach it. But if there was a wormhole connecting the star and your front porch, you could walk to the star in a few steps. 
    
    While worm holes do not violate any known laws of physics, scientist think that these phenomenas, if they existed, would require unrealistic amounts of negative energy-a bizarre state of matter that has less than zero energy. 

What is a white hole?

    A white hole is a theoretical tool that "balances out" the concept of a black hole; no astronomer has actually observed this phenomenon. The equations of general relativity are symmetrical, and an idealized black hole (a region of very dense space from which nothing can escape) requires a corresponding white hole to keep it balanced in space-time (space-time refers to the medium in which matter and energy exist). While a black hole sucks in everything, even light, a white hole spits everything out. 

What is the grandfather paradox?

    The grandfather paradox is used to illustrate how time travel could violate causality (cause and effect). Here's the premise: suppose you built a time machine and used it to travel seventy years back in time. In theory, you could then kill your grandfather before he met your grandmother, and change the course of history, 

    The paradox is that if your grandfather died before he met your grandmother, then you would never have been born. But if you were never born, you could not have gone back in time to prevent your own birth. 

What is a superconductor? 

    Certain materials that display no resistance to the flow of electric current are called superconductors. They can carry enormous amounts of current, and the currents can persist for long periods of time (years) without loss of energy. They are an essential component of particle accelerators, where powerful electromagnets need extremely large amounts of current.
Superconductivity can only be achieved if the super-conducting material is below a certain critical temperature. Before 1986, the working temperature for a superconductor could not exceed 23.2K (-249.8°C), which required the use of liquid helium, an expensive and inefficient coolant. But due to the research at several universities, ceramic metal-oxide compounds were found to be superconductivity at much higher temperatures. Instead of liquid helium, liquid nitrogen became the preferred coolant.

What is Occam's razor?

    Occam's razor is a philosophical rule for choosing between competing theories. It states that if two theories predict and explain phenomena to the same degree of accuracy, then the theory that is simpler is more valid. Moreover, additional aspects of a theory that do not lend it greater predictability are unnecessary and should be stripped away.

What is the "Big bang" theory?

    The Big bang theory proposes that the universe began in an explosion approximately 10 billion or more years ago, and that it is still expanding from that cataclysm. Big bang theorists believe that the various particles of matter today were produced within the first few minutes of the big bang, when energy and extremely high temperatures formed the elements in the early universe. 
Recent evidence indicates that hydrogen and helium would have been the primary products of the big bang, with heavier elements, such as oxygen and carbon, produced later within stars. As the universe expanded, the hydrogen and helium cooled and condensed into stars and galaxies.